The present invention relates to a method of improving the criticality safety in a liquid-liquid extraction process for recycling spent nuclear fuel and/or breeder reactor materials, particularly those processes in which operating parameters vary from normal, or prescribed, limits.
One of the best known liquid-liquid extraction processes for recycling spent nuclear fuel and/or breeder reactor materials is the so-called PUREX process (e.g: R. I. Stevenson, P. E. Smith: Reactor Handbook (1961), Vol.II, p. 107: "Aqueous Separation") in which uranium and plutonium are separated from the accompanying fission, corrosion and activation products, neptunium, other transuranium elements and other contamination products by extraction from the nitric acid dissolver solution of the fuel or breeder reactor elements by means of the organic extraction agent tri-n-butyl phosphate (TBP), dissolved in an organic solvent or diluant. The separation is made possible by the differences in the distribution coefficients of the individual chemical species in TBP. The usual complexation strengths can be illustrated, for example, by the following sequence: ##EQU2##
Multistage mixer settlers and centrifugal extractors as well as pulsed sieve plate columns are employed as countercurrent extraction apparatus. In the steps involving extraction of the valuable materials and washing away of unwanted fission products, for example in columns HA, HS and TS of FIG. 1, the separation of the poorly extractable fission products is supported by various measures:
1. Particularly for the better separation of Ru, washer HS is operated at a higher HNO.sub.3 concentration (at about 4 mol HNO.sub.3 /1).
2. For the better separation of Zr, the washer marked TS is operated at a lower HNO.sub.3 concentration than for the extraction (at about 1 mol HNO.sub.3 /l).
3. Generally, saturation of the extraction agent with the easily extractable UO.sub.2.sup.2+ and PU(IV) ions at the highest possible level is desirable to keep the amount of free TBP, which is able to form complexes with fission products, as low as possible.
4. The nitric acid concentration in the aqueous feeder solution HAF before extraction of the values uranium and plutonium is set to about 3 mol/l because this concentration represents a compromise for the separation of Zr and Ru in the first extractor HA. With a high acid concentration, Zr is also extracted while Ru is extracted into the organic phase only at a lower acid concentration.
With the desired high saturation of the extraction agent, the traditional process is operated in a narrow range because even small changes in external parameters, such as, for example, those listed below, may cause losses of U and especially also of Pu into the aqueous waste solution HAW (see FIG. 2):
slow-down of extraction agent flow, HAX;
increase in the flow of feed solution, HAF;
an HNO.sub.3 concentration in the feed solution, HAF, which is too low and not within recommended limits;
U and PU concentrations in the feed solution, HAF, which are not within recommended limits.
It is a particular drawback that, due to the lower complex formation, the more valuable and also more toxic Pu runs into the aqueous waste solution HAW. In the extreme case, this may raise criticality problems in the subsequent concentration and processing process of the aqueous waste solution. For that reason, the Pu content in the HAW must be monitored constantly and accurately in the industrial process by means of costly operational analyses. A complicated analysis vessel system installed in a critically safe configuration ensures that charges of too high a Pu concentration break through into the concentration evaporator. An automatic and reliable monitoring instrument system for this purpose has not yet become available.
Another problem touching on criticality safety relates to the peak-like accumulation of Pu occurring in all types of extraction apparatus during operation not outside normal parameters. In the extreme case, such accumulations may reach peak concentrations of several tens of grams of Pu per liter in the aqueous phase, as indicated by the corresponding curves in FIGS. 2 and 3 calculated for light water reactor and fast breeder reactor fuel components. Experimental tests regarding this problem (W. Ochsenfeld, H. Schmieder, S. Theiss, KfK-Bericht [KfK Report] 911, pages 15-16, 1970) have occupied those skilled in the art since about the early seventies. For example, just recently, complicated and expensive extraction test stands have been set up to examine this problem in the Federal Republic of Germany and also in other countries (I. Kobayashi et al, Japan Atomic Energy Research Institute, Report JAERI-M 85-152, 1985).
An additional problem occurring with Pu accumulations is the formation of a third, dense, organic phase in the extraction apparatus. This danger occurs, for example, with Pu concentrations of 25 g/l organic phase and higher. Depending on conditions, this corresponds to an aqueous Pu concentration of 10 or several 10 grams per liter, i.e. a concentration as may develop with the above-mentioned Pu accumulations in operation outside normal limits. The third heavy organic phase may contain Pu in concentrations of 50 g/l and more. Because the organic phase is denser than the aqueous phase, uncontrolled accumulations in the extraction apparatus must be expected and this must be considered in the context of criticality safety. Criticality safety within the extraction apparatus is ensured by geometric measures and/or, depending on the size of the apparatus, by the use of homogeneous or heterogeneous neutron absorbers (H. Schmieder et al, KfK-Bericht [KfK Report] 2940, page 144, 1980). In addition, safety instruments (neutron monitors) are often provided to permit detection of Pu accumulations as early as possible. It is obvious that these measures, together with the required efficiency checks, are complicated and expensive.
A further drawback of the traditional PUREX process is the unsatisfactory low separation of Np during the first extraction. Generally, only about 10% of the Np contained in the nuclear fuel is removed into the HAW. Thus, complicated measures for its separation become necessary in the purification cycles.